The present invention relates generally to the treating of wells, and, in a more particular embodiment, to a method and apparatus for accurately placing a well treatment fluid in a a hydrocarbon-producing well. The present invention relates to a downhole device that is capable of detecting the position of a fluid interface so that a well treatment can be placed with greater along hole depth precision in a given well.
A variety of well treatments using treating fluids are performed in the completion and stimulation of oil and gas wells. These treatments include, but are not limited to: well remediation, non-damaging kill fluids, water abatement with polymerizing gels, water abatement by relative permeability reduction, gas abatement with foams and gelled foams, clay stabilization, scale inhibition, wax deposition and removal, and hydraulic fracture treatments. Depending on the purpose of the treatment, the treating fluid may or may not be applied along the entire length (depth) of the well. In instances where the purpose of the treatment is to cause a change in only a localized region along the depth of the well, it is desirable to limit contact between the treating fluid and the rest of the formation.
For example, cementing treatments are carried out in the construction and repair of wells utilizing a cement composition as the treating fluid. In forming a cement composition, a hydraulic cement is normally mixed with water and other additives to form a pumpable cement composition that is placed in a subterranean zone that is penetrated by well bore. After placement in the zone, the cement composition sets into a hard, substantially impermeable mass within the zone.
The most common cementing treatment or operation performed in the construction of a well is primary cementing, wherein a metal pipe string, such as casing or a liner, is placed in the well bore and bonded therein by cement. Other cementing treatments utilized in wells are usually remedial in nature. For example, a cement composition is often squeezed into cracks or openings in pipe disposed in the well bore, in the cement sheath in the annulus between the pipe and the well bore, and in other similar locations and allowed to set, so that the cracks or openings are plugged.
High viscosity well treating fluids are also utilized in well completions and in the stimulation of formations penetrated by the well bore to enhance the production of oil and gas therefrom. The most common of such treating fluids are high viscosity gelled fluids that are utilized in completion treatments, such as in forming gravel packs, and stimulation treatments, such as hydraulic fracturing.
Hydraulic fracturing is performed by injecting a high viscosity fluid through the well bore into the subterranean formation that is to be fractured and applying sufficient fluid pressure on the formation to cause its breakdown and the production of one or more fractures therein. A fracture proppant material, such as sand or other particulate material, is usually suspended in the high viscosity fracturing fluid so that the proppant material is carried into the fractures and deposited therein. When pressure on the fractured formation is released, the fractures are propped open by the proppant material therein.
Another instance in which it is desired to treat a specific portion of the formation is in wells that have a significant water production. While the oil well is usually completed so as to draw from an oil-bearing zone, in wells where there is a water bearing zone adjacent to the oil zone or there is a water drive mechanism, the water often flows into the well by way of natural fractures, coning, bottom or edge water encroachment, channels behind pipe and high permeability streaks in the formation. In the production of such wells, the ratio of water to oil recovered may become so high that the cost of producing the water, separating the water from the produced oil and disposing of the water represents a significant economic loss.
It is known to use cross-linking aqueous polymer solutions to reduce the production of water from such wells. According to common practice, an aqueous polymer solution is pumped into the water bearing portion of the formation. The polymer solution then crosslinks so that it forms a stiff gel. The gel plugs the natural fractures, intergranular porosity, channels and high permeability streaks through which water would otherwise flow into the wellbore. An example of such process can be found in U.S. Pat. No. 5,181,568, hereby incorporated herein by reference.
Because a water reduction treatment using an aqueous polymer solution results in the permanent permeability reduction of the formation, it is imperative that the permeability in the oil zone is not reduced as this will potentially destroy all oil production. Furthermore, the relatively large volumes of aqueous polymer solution required for performing the heretofore used polymer water reduction treatments causes the treatments to be very expensive. Thus, there is a need for an improved method of selectively placing these permeability reducing treatments in a subterranean oil bearing formation that has started to produce water without incurring the above mentioned problems and high cost.
In all of the various completion and stimulation treatments where a treating fluid is introduced into a subterranean zone penetrated by a well bore, it is difficult to confirm whether and to what degree the treating fluid has entered the desired subterranean zone. In particular, when it is desired to apply a treatment to only a specific portion of the formation, it is difficult to direct the treatment to the specific portion. While it is possible to treat the entire well for the purpose of treating the specific portion, it is sometimes difficult to ensure that the subject zone has received any treatment at all. For example, when the purpose of a treatment, such as acidizing, is to increase the permeability of a relatively impermeable layer in the formation, the low permeability of that layer prior to treatment will limit effectiveness of the treatment on that stratum. On the other hand, because of their relative permeability, the other portions of the formation, which were already sufficiently permeable, are likely to be contaminated or otherwise affected by the treatment fluid.
It is possible to isolate the formation layer that is to be treated so that the treating fluid only contacts that layer. This may require the use of packers above and below the layer in question. These packers can be run on coiled tubing or standard tubulars. The packer is placed between the casing and tubular. Placement of such packers is time-consuming, increases the complexity and is expensive, with the packers themselves adding to the cost of the operation. Also, packers have coherent technical limitations and may cause problems going through and coming back through restrictions.
Partly in response to this problem, significant time and energy has gone into the development of methods for detecting the locations of a well treating fluid as it is being introduced into a well. In one common practice, a radioactive tracer material is included in the protection fluid or treating fluid. During the placement of the protection fluid or treating fluid containing the radioactive tracer, an instrument that detects radioactivity is included on the coiled tubing or work string and is used to determine the location or locations of the protection fluid or treatment fluid.
Radioactive tracers are expensive and are considered hazardous. They and the fluids containing them must be handled and disposed of in accordance with the laws and rules relating to hazardous materials. These measurements of fluid placement, while somewhat accurate, are not entirely precise. Finally, even if packers are used to isolate a zone that is going to be treated, this may not help in certain kinds of completion such as gravel packs, where there is a fluid communication path through the gravel pack jacket, or in cases where there is a bad cement bond and a channel behind pipe.
Thus, there is a need for a relatively inexpensive, effective method of accurately placing a treating fluid in contact with a desired formation layer. It is preferred that the system not involve the use of radioactive tracer materials or other hazardous materials that require disposal of in a special manner.
The present invention provides a relatively inexpensive, effective method for accurately placing a treating fluid in contact with a desired formation layer. The present system does not involve the use of radioactive tracer materials or other hazardous materials that must be disposed of in a special manner, nor does it require the use of packers. Because it allows much more accurate placement of well treatments, the present invention provides an improved method for selectively reducing the permeability of water bearing subterranean formations at relatively low cost and without damaging the oil-producing zones of the formation.
More particularly, an embodiment of the invention includes a method for accurately placing a well treatment fluid in a well, comprising: pumping a first fluid into a first part of the well until an interface is formed between the first fluid and a second fluid; extracting information regarding at least one fluid property of the first and second fluids with first and second sensors positioned in the first and second fluids respectively; and exchanging information between the first and second sensors and a telemetry unit.
The invention also includes a downhole tool for positioning a fluid interface in a well bore, comprising: first and second sensors, the spacers being spaced apart such that they span the fluid interface; the first sensor measures a first fluid property and the second sensor measures a second fluid property; a first fluid port on the same side of the first sensor as the spacer and in fluid communication with a first fluid flow line; and a second fluid port on the opposite side of the first sensor as the spacer and in fluid communication with a second fluid flow line.